1. Field
The invention relates to a preferred method for fabricating an improved dielectric filled ferrite toroid.
2. Prior Art
Phased array radar systems utilize, inter alia, phase shifters generally of a ferrite material, to vary the phase, direction of the radar beam, etc., without moving the antenna, to provide accordingly an electrical radar scan. The preferred phase shifter for such application employs a rectangular ferrite toroid, having a rectangular insert of dielectric material disposed therein, which is installed within the waveguide system.
The dielectric filled toroids are presently manufactured by techniques which are not readily adapted to high speed, mass production techniques. Typical of such manufacturing processes is the procedure wherein the ferrite part is pressed in the green ferrite state about a selected, removable, steel mandril. The mandril is removed and the ferrite part is then fired to dimensions. A machined dielectric insert is subsequently disposed in the slot formed by the mandril. As may be seen, the pressing technique supra generally requires two pressing steps; viz, a portion of the ferrite material is first partially pressed, the mandril is placed upon the partially pressed ferrite, additional ferrite material in the green state is added, and the composite ferrite part is then pressed in one piece to complete the two step pressing process. It is necessary to fire the ferrite to provide the required magnetic characteristics, which shrinks the ferite material to preselected dimension, as commonly known in the art. Accordingly, the two step process requires starting with an oversize toroid of selected dimensions such that firing the toroid shrinks to the required final dimensions.
The two step pressing process briefly described above provides ferrite toroids of substantial imperfections, that is, provides toroids wherein the remanent magnetization is not consistently repeatable due to dimensional variations in each toroid manufactured. Accordingly, the pressing technique provides a very low yield of acceptable product of the order of less than 50 percent. This, in turn, increases the expense of the normally expensive ferrite phase shifters. In addition, the acceptable phase shifters fabricated by the pressing technique have varying remanent magnetizations and must be individually fitted to use in the radar system.
In addition, when performing the pressing technique, unequal pressures are exerted at various points of the toroidal cross-section which, when fired, generates a toroid of varying densities thoughout its cross-section as a result of the non-uniform distribuion of the applied force. This in turn causes warpage, twisting, or bowing of the toroid, or causes the ferrite toroid to crack and thus fail, generally at the corners of the cross-section. In ferrite toroids of any substantial length, the warpage or bowing in turn gives rise to a thinning of some portion of the wall cross-section, which results in a reduction of the remanent magnetization of the toroid. Insertion of the machined dielectric insert forms air gaps between the insert and the ferrite, and results in an unpredictable variation of the phase shifters insertion phase. Accordingly, the air gaps in prior art toroids are filled with a high dielectric constant material such as, for example, an epoxy material impregnated with selected dielectric material. This procedure produces toroids with unrepeatable remanent magnetization characteristics, and leads to catastrophic failure due to the thermal expansion mismatch between the filler and ferrite.
An alternate prior art manufacturing technique utilizes an isostatic pressing technique, wherein the ferrite material is pressed about a removable mandril utilizing a plastic bag to confine the product, and a fluid disposed about the plastic bag. Pressure imparted to the fluid conforms the ferrite to roughly the desired shape about the mandril.
Both the above pressing techniques have serious faults in that they are economically unfeasible, due largely to the fact that there is a low rate of yield. For example, in the first technique severe difficulty is encountered, as previously mentioned, in maintaining all interior dimensions to the required tolerances of the order of for example .+-.0.001 inches due to variable shrinkage, warpage and other uncontrollable phenomena inherent in the process. The technique also is generally relegated to manual operation, since any automatic press capable of performing the required functions is relatively sophisticated and thus expensive.
The second, isostatic, pressing technique has problems similar to those described above, while also yielding ferrite toroids which have considerable material excess in the finished product, requiring accordingly extensive machining to provide the required external dimensional tolerances.
It follows accordingly, that it would be highly desirable to fabricate the ferrite toroid in two parts, whereby the parts may be readily machined to exact dimensions and accurately assembled about a dielectric material of equally exact dimensions. However, the assembly of two ferrite parts along their lengths is obviously accompanied by the formation of undesirable longitudinal air gaps between the mating surfaces of the two parts. Such gaps in turn cause a deterioration of the electromagnetic characteristics of the toroid, i.e. a shearing in the magnetic hysteresis loop, and an associated indeterminable deterioration of the performance of the ferrite toroid in the waveguide system. Thus prior art manufacturing techniques have been limited to the various pressing techniques, and the associated low yield fabrication of relatively inaccurate, one-piece toroids about a dielectric insert.